1. Technical Field
The present invention relates to an optical head device for writing optical information on or reading out optical information from an optical recording medium such as a CD or a DVD, a diffracting element suitable therefor, and a process for producing the diffracting element.
2. Background Art
Heretofore, to write optical information on an optical recording medium such as an optical disk or a magneto-optical disk such as a CD or a DVD, or to read out optical information from an optical recording medium, an optical head device provided with e.g. a diffracting element for introducing signal light as reflected light from the optical recording medium to a light-receiving element as a photodetector, has been used.
In order to increase the go and return efficiency of light of a diffracting element to at least the go and return efficiency of 10% of unpolarization type independent of the polarization direction of light, it has been proposed to use e.g. a hologram beam splitter of polarization type wherein the go and return efficiency varies depending upon the polarization direction of light, as the diffracting element.
The hologram beam splitter of polarization type has such a structure that an isotropic diffraction grating made of an isotropic optical material having a refractive index equal to the ordinary refractive index or the extraordinary refractive index of a birefringent optical material such as liquid crystal, formed on e.g. a glass substrate, and a facing substrate provided with an alignment film having alignment treatment applied thereto, are arranged so as to face each other and subjected to thermal contact bonding by means of a sealing material, and a birefringent optical material such as liquid crystal is filled in the inside.
In the case of using the hologram beam splitter of polarization type, the polarization direction of light when passing through the hologram beam splitter is rotated by 90 degree between the going path (the direction from the light source to the optical recording medium surface) and the returning path (the direction from the optical recording medium surface to the light source side and the light-receiving element side) by combining the hologram beam splitter with a quarter-wave plate.
Since an optical disk is rotated at a high speed, with respect to reflected light from the optical disk, having partial birefringence of at least one part, the polarization direction is not constant and varies with time, and the polarization direction in returning path relative to the polarization direction in going path departs from the 90 degree, and reflected light wherein the degree of the departure in the polarization direction is completely unclear, will come back.
In the worst case, the reflected light from the optical disk may come back in the same polarization direction as the going path. In this case, the hologram beam splitter of polarization type can hardly diffract the reflected light, whereby information in the optical disk can not be read out by the light-receiving element.
Under these circumstances, it is a first object of the present invention to provide a diffracting element and an optical head device for reading out information even from an optical recording medium having birefringency.
On the other hand, a diffracting element using a thin film of polymer liquid crystal has conventionally been prepared as follows. Namely, alignment treatment is applied to a transparent substrate surface of e.g. glass, this transparent substrate surface is thinly coated with liquid crystal, followed by curing by photopolymerization to obtain a thin film of polymer liquid crystal. A grating having a cross-sectional shape of projections and recesses is formed on the thin film of polymer liquid crystal by applying e.g. dry etching, and these projections and recesses are filled with an optical isotropic medium.
However, it is difficult to stabilize the alignment condition obtained when the substrate having the alignment treatment applied thereto is coated with the liquid crystal in the monomer state before the polymerization, in production of the thin film of polymer liquid crystal. If the alignment state is not stable, the substantial birefringent state after the polymerization may change, the refractive index may not be stable and no desired diffraction efficiency may be obtained, and a diffracting element can not be obtained with a high yield, such being problematic.
Further, it is also difficult to control the thickness of the film of polymer liquid crystal, and if the thickness of the film varies, the diffraction efficiency will vary, thus leading to decrease in the production yield of the diffracting element.
It is a second object of the present invention to provide a process for producing a diffracting element, in which the refractive index is stable as the thickness of the film is constant, whereby the desired diffraction efficiency can be obtained.
The present invention provides a diffracting element comprising an optical material forming a diffraction grating having a cross-sectional shape of projections and recesses, and a separate optical material filled at least in the recesses of the diffraction grating, wherein at least one of the above two types of optical materials shows birefringence, and the other optical material has at least one refractive index which is different from both the ordinary refractive index and the extraordinary refractive index of said one optical material showing birefringence.
The present invention further provides a process for producing a diffracting element having a grating having a cross-sectional shape of projections and recesses formed on a thin film of polymer liquid crystal, and having the grating having a cross-sectional shape of projections and recesses filled with an isotropic medium, which process comprises applying alignment treatment to a facing surface of at least one of two substrates facing each other, sandwiching spacers and liquid crystal to be a thin film of polymer liquid crystal between the substrates facing each other, having the liquid crystal aligned and cured to obtain the thin film of polymer liquid crystal, and then taking at least one substrate off and forming a grating having a cross-sectional shape of projections and recesses on the thin film of polymer liquid crystal, and filling at least the recesses of the grating having a cross-sectional shape of projections and recesses with an isotropic medium.
The present invention further provides an optical head device comprising a light source, a diffracting element for passing light emitted from the light source therethrough and changing the traveling direction of reflected light reflected and coming back from an optical recording medium, and a photodetector for detecting information of the reflected light having the traveling direction changed by the diffracting element, wherein the above-mentioned diffracting element is used as the diffracting element.